Polarity in semiconductor materials arises from their crystal structure, and potentially from lattice distortion of the crystal lattice caused in particular by strained crystal lattices in semiconductor heterostructures.
Examples of polar semiconductors are compound semiconductors of hexagonal crystal structure, which have a polar axis along their c-direction and compound semiconductors of zincblende structure, which have a polar axis along their [111] direction. The following introduction will concentrate on hexagonal polar semiconductors as a non-limiting example.
Compound semiconductors of hexagonal crystal structure have constituent atoms arranged in a wurtzite structure. One non-limiting example of a group of polar semiconductor materials are compound semiconductors of hexagonal crystal structure such as group-III nitride semiconductors like GaN, AlN, AlGaN, InGaN, InAlN, or InGaAlN. They will also be referred to in short as group-III nitrides and as nitride semiconductors herein. Group-III atoms and nitrogen atoms are arranged in respective hexagonal sub-lattices. An extended unit cell of such materials is hexagonal and has a polar crystal axis parallel to the c-direction of the crystal lattice, which is called the c-axis. The c-axis points in a direction perpendicular to a (0001) plane of the hexagonal crystal lattice. The (0001) plane is called the C plane. Due to the hexagonal crystal structure, the C plane of group-III nitride semiconductor materials may terminate in one of two different configurations. A first configuration is called group-III-face (or Ga face, Al face, depending on the material) and has nitrogen (N) atoms bonded to three group-III atoms towards the surface. It is also referred to as the III-polar configuration. A second configuration which is known as N face and has a respective nitrogen atom bonded to a single group-III atom towards the surface. It is also referred to as the N-polar configuration. These two configurations should not be confused with modes of surface termination. Either configuration can be terminated on the surface with either group-III atoms or nitrogen atoms.
Hexagonal nitride semiconductor materials have a strong electrical polarization field along the c-axis. Such spontaneous polarization fields exist even in relaxed layers. A discontinuity of the electrical polarization at interfaces between layers of different material composition results in giant electric fields, which are known to have strong effects on the characteristics of device performance. Additional polarization is created in heteroepitaxial layers by a strained crystal lattice. In particular, the strong electric fields in the range of several MV/cm are responsible for substantial band bending effects, and for a spatial separation of wave functions of electrons and holes in quantum confinement structures such as quantum wells, quantum wires and quantum dots. A reduced overlap in the wave functions of electrons and holes is responsible for a reduced efficiency of light emission in semiconductor light emitter devices based on nitride semiconductors. Furthermore, the band bending and resulting spatial separation of electrons and holes result in a red shift of light emission in comparison with the so-called flat-band case, in which no electric fields are present.
DE 199 53 839 A1 is concerned with overcoming such disadvantages of a strong electrical polarization field that hexagonal nitride semiconductor materials have along the c-axis. The solution proposed by DE 199 53 839 A1 is to grow nitride semiconductor materials with a hexagonal crystal structure such that the c-axis of the hexagonal crystal structure is oriented parallel to a substrate surface. This concept requires use of an “exotic” substrate material that is not commonly used in semiconductor technology.